It's probably because it is lower than you are used to, a so called "false hypo".
It's unlikely (although not impossible) that you'll get a real hypo as a T2 on metformin only. It's probably a good sign - it means that you are bringing your blood sugar levels to more normal range. It tends to get better once you've done it for a while.
I suggest you buy a cheap meter and a few strips so that you can make sure for yourself.